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Polymer Composite Electrolytes and Their Interfacial Engineering for Lithium Batteries
thesisposted on 01.05.2021, 00:00 by Ramin Rojaee
Although solid-state materials are growing attention for their application in energy storage devices, poor electrochemical properties including low ionic conductivity, high interfacial resistance, limited transference number and low cycle life are among major challenges to commercialize such electrochemical systems. Owing to compelling physicochemical and structural properties, in recent years two-dimensional (2D) materials have emerged as promising candidates to address the challenges in lithium-based batteries (LIBs). A comprehensive overview of the role and potentials of 2D materials to overcome the slow kinetics and limited electrochemical properties of the Li batteries is discussed. Part of this research was focused in utilizing phosphorene 2D material as an electrolyte additive and understanding the Li+ ion conduction mechanism in this composite electrolyte. Here, we report a novel quasi-solid Li+ ion conductive nanocomposite polymer electrolyte containing black phosphorous (BP) nanosheets. The developed electrolyte was successfully cycled against Li metal (over 550 h cycling) at 1 mA.cm-2 at room temperature. The cycling overpotential dropped by 75% in comparison to BP-free polymer composite electrolyte indicating lower interfacial resistance at the electrode/electrolyte interfaces. This study revealed that the coordination number of Li+ ions around TFSI- pairs and ethylene oxide (EO) chains decreases at the Li metal/electrolyte interface, which facilitates the Li+ transport through the polymer host. In addition, interfacial engineered electrode/electrolyte design was introduced to address slow Li+ ion transport an undesired degradation reaction at the electrode/self-standing polymer electrolyte interfaces. This work represents an in-situ UV crosslinked integrated network of electrolyte and active electrode materials with controlled interfacial resistance and voltage polarizations. This method was shown to provide a uniform morphology of composite polymer electrolyte with a low thickness of 20-40 um. The interfacial engineered self-standing polymer batteries decreased the interfacial resistance as high as seven folds compared to conventional self-standing polymer batteries. This modification has led to promising cycling results and provided 70 % capacity retention over 100 cycles at high current densities of 170 mA.g-1. This work offers novel methods to tune the bulk and interfacial ionic conductivity of polymer electrolytes that may lead to a new generation of lithium polymer batteries with high ionic conduction kinetics and stable long-life cycling.